add Marshaler interface

[kovetskiy]

Hi @BurntSushi, I've noticed that there is no nice way to handle Marshal/Encode operation unless implementing encoding.TextMarshaler interface, which can't be used if struct should be also marshalable to JSON (json libraries also looks for encoding.TextMarshaler),

so I've added Marshaler interface with following signature:

type Marshaler interface {
    MarshalTOML() ([]byte, error)
}

Also I've added various tests for this feautre.

[kovetskiy]

@BurntSushi ping

[cespare]

If a user-defined type is a Marshaler and a TextMarshaler, the former should take precedence.

[kovetskiy]

@cespare I've force pushed changes with Marshaler precedence.

[kovetskiy]

@BurntSushi @cespare ping

[kovetskiy]

@BurntSushi @cespare any reaction?

[cespare]

Fixes #76
Related to #63

[cespare]

@kovetskiy sorry, been busy. I'll try to take a look this weekend.

[kovetskiy]

@cespare @BurntSushi ping

[kovetskiy]

@cespare @BurntSushi ping

[tucnak]

jeez @kovetskiy it must hurt lol

[kovetskiy]

@tucnak yeah, such maintainers

[cespare]

You're already calling rv.Interface().(Marshaler) in the caller; pass in a Marshaler rather than a reflect.Value and avoid the duplicate work.

[cespare]

I was hoping that adding Marshaler could be a solution for #64, #75, and other cases where there's no way to output a particular encoded value.

For example, I was thinking that if you have a MarshalTOML method like

func (T) MarshalTOML() ([]byte, error) {
    return []byte(`# hello
a = """
1
2
3"""
b = '\n'`), nil
}

then that would yield a TOML encoding with a comment, a multi-line string, and a literal string:

# hello
a = """
1
2
3"""
b = '\n'

but since your approach is to decode and then encode the value, we instead get

a = "1\n2\n3"
b = "\\n"

I agree that we need to decode the value to check that it's valid TOML, but I think we should emit the original encoding. WDYT?

[langston-barrett]

@kovetskiy Any plans to address the comments here?